[llvm-dev] About LoopDeletion and infinite loops ... again! (RFC?)

[Hal Finkel via llvm-dev]

On 09/29/2017 10:02 PM, Marcello Maggioni via llvm-dev wrote:
> I see the usecase for mixed language compilation (that’s probably why you fancy something like the side-effect thing instead right?)

It's also about not having parameterized semantics for the IR. I'd 
certainly find that undesirable. We could make it part of datalayout, or 
similar, but that has problems with mixed-language compilation. It also 
has problems with C, where we generally get to assume that loops 
terminate, but loops with a constant controlling condition (at the 
source level) can be infinite. Whether or not something is a constant 
condition at the source level is something that we need the frontend to 
mark only for specific loops.

>
> BTW if the other proposal passes can we basically assume that if a loop doesn’t have the sideeffect intrinsic in it is then removable?

If it does not have the sideeffect intrinsic, or some other actual side 
effect (or atomic, etc.), then yes.

>
> That patch seems to suggest that in its current state llvm is mostly broken for languages that consider all infinite loops as unremovable ... (but I didn’t read all the discussion)

As I understand it, it is broken in places, but we also don't take 
advantage of assuming that loops terminate as much as we could (assuming 
we fully commit to that assumption).

  -Hal

>
> Marcello
>
> Sent from my iPhone
>
>> On Sep 29, 2017, at 7:43 PM, Davide Italiano <davide at freebsd.org> wrote:
>>
>> On Fri, Sep 29, 2017 at 7:17 PM, Marcello Maggioni via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>>> Hello!
>>>
>>> I read a bunch of discussions about the matter on this very mailing-list
>>> that are relatively recent or relatively old and I couldn’t find much
>>> agreement on the matter, so … here again :D
>>>
>>> LoopDeletion and infinite loops …
>>>
>>> Currently LoopDeletion bails if non-detectable trip count loops are
>>> encountered and that’s fine, there are languages where infinite loops
>>> without side effects cannot be removed.
>>>
>>> If I read the C++ spec right though N4527 (sec 1.10)  point 27 says:
>>>
>>> 27  The implementation may assume that any thread will eventually do one of
>>> the following:
>>>
>>> 27.1 — terminate,
>>>
>>> 27.2 — make a call to a library I/O function
>>>
>>> (27.3)— read or modify a volatile object, or
>>>
>>> (27.4)— perform a synchronization operation or an atomic operation.
>>>
>>> [Note: This is intended to allow compiler transformations such as removal of
>>> empty loops, even when
>>>
>>> termination cannot be proven. — end note ]
>>>
>>>
>>> So, the spec seems to be explicitly calling out that removing infinite loops
>>> is ok unless they do any of the things mentioned there (at least that is my
>>> interpretation, is that correct?).
>>>
>>>
>>> If that is the case we could be missing out for languages that have such a
>>> behavior (and in particular in C++).
>>>
>>> I was wondering how it would be viewed the possibility of adding a flag to
>>> loop deletion that allows the removal of loops with loop counts that are not
>>> SCEVable.
>>>
>> You probably have seen it, but, for reference
>> https://reviews.llvm.org/D38336 (which goes in the exact opposite direction).
>>
>> I don't necessarily fancy the idea of having a flag, instead, maybe,
>> the frontend could emit enough information to tell the optimizer
>> whether it is safe to remove the loop?
>>
>> Thanks,
>>
>> --
>> Davide
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory